Drill pipe explosive severing tool

ABSTRACT

A pipe severing tool is arranged to align a plurality of high explosive pellets along a unitizing support structure whereby all explosive pellets are inserted within or extracted from a tubular housing as a singular unit. Electrically initiated exploding wire detonators (EBW) are positioned at opposite ends of the tubular housing for simultaneous detonation by a capacitive firing device. The housing assembly includes a detachable bottom nose that permits the tool to be armed and disarmed without disconnecting the detonation circuitry. Because the tool is not sensitive to stray electrical fields, it may be transported, loaded and unloaded with the EBW detonators in place and connected.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to the earthboring arts. Moreparticularly, the invention relates to methods and devices for severingdrill pipe, casing and other massive tubular structures by the remotedetonation of an explosive cutting charge.

[0003] 2. Description of Related Art

[0004] Deep well earthboring for gas, crude petroleum, minerals and evenwater or steam requires tubes of massive size and wall thickness.Tubular drill strings may be suspended into a borehole that penetratesthe earth's crust several miles beneath the drilling platform at theearth's surface. To further complicate matters, the borehole may beturned to a more horizontal course to follow a stratification plane.

[0005] The operational circumstances of such industrial enterpriseoccasionally presents a driller with a catastrophe that requires him tosever his pipe string at a point deep within the wellbore. For example,a great length of wellbore sidewall may collapse against the drillstring causing it to wedge tightly in the well bore. The drill stringcannot be pulled from the well bore and in many cases, cannot even berotated. A typical response for salvaging the borehole investment is tosever the drill string above the obstruction, withdraw the freed drillstring above the obstruction and return with a “fishing” tool to freeand remove the wedged portion of drill string.

[0006] When an operational event such as a “stuck” drill string occurs,the driller may use wireline suspended instrumentation that is loweredwithin the central, drill pipe flow bore to locate and measure the depthposition of the obstruction. This information may be used to thereafterposition an explosive severing tool within the drill pipe flow bore.

[0007] Typically, an explosive drill pipe severing tool comprises asignificant quantity, 800 to 1,500 grams for example, of high orderexplosive such as RDX, HMX or HNS. The explosive powder is compactedinto high density “pellets” of about 22.7 to about 38 grams each. Thepellet density is compacted to about 1.6 to about 1.65 gms/cm³ toachieve a shock wave velocity greater than about 30,000 ft/sec, forexample. A shock wave of such magnitude provides a pulse of pressure inthe order of 4×10⁶ psi. It is the pressure pulse that severs the pipe.

[0008] In one form, the pellets are compacted at a production facilityinto a cylindrical shape for serial, juxtaposed loading at the jobsiteas a column in a cylindrical barrel of a tool cartridge. Due to weightvariations within an acceptable range of tolerance between individualpellets, the axial length of explosive pellets fluctuates within a knowntolerance range. Furthermore, the diameter-to-axial length ratio of thepellets is such that allows some pellets to wedge in the tool cartridgebarrel when loaded. For this reason, a go-no-go type of plug gauge isused by the prior art at the end of a barrel to verify the number ofpellets in the tool barrel. In the frequent event that the tool must bedisarmed, the pellets may also wedge in the barrel upon removal. Anon-sparking depth-rod is inserted down the tool barrel to verifyremoval of all pellets.

[0009] Extreme well depth is often accompanied by extreme hydrostaticpressure. Hence, the drill string severing operation may need to beexecuted at 10,000 to 20,000 psi. Such high hydrostatic pressures tendto attenuate and suppress the pressure of an explosive pulse to suchdegree as to prevent separation.

[0010] One prior effort by the industry to enhance the pipe severingpressure pulse and overcome high hydrostatic pressure suppression hasbeen to detonate the explosive pellet column at both endssimultaneously. Theoretically, simultaneous detonations at opposite endsof the pellet column will provide a shock front from one end collidingwith the shock front from the opposite end within the pellet column atthe center of the column length. On collision, the pressure ismultiplied, at the point of collision, by about 4 to 5 times the normalpressure cited above. To achieve this result, however, the detonationprocess, particularly the simultaneous firing of the detonators, must betimed precisily in order to assure collision within the explosive columnat the center.

[0011] Such precise timing is typically provided by means of milddetonating fuse and special boosters. However, if fuse length is notaccurate or problems exist in the booster/detonator connections, thecollision may not be realized at all and the device will operate as a“non-colliding” tool with substantially reduced severing pressures.

[0012] The reliability of state-of-the-art severing tools is furthercompromised by complex assembly and arming procedures required at thewell site. With those designs, regulations require that explosivecomponents (detonator, pellets, etc.) must be shipped separately fromthe tool body. Complete assembly must then take place at the well siteunder often unfavorable working conditions.

[0013] Finally, the electric detonators utilized by state-of-the-artsevering tools are not as safe from the electric stray currents and RFenergy points of view, further complicating the safety procedures thatmust be observed at the well site.

SUMMARY OF THE INVENTION

[0014] The pipe severing tool of the present invention comprises anouter housing that is a thin wall metallic tube of such outside diameterthat is compatible with the drill pipe flow bore diameter intended foruse. The upper end of the housing tube is sealed with a threaded plughaving insulated electrical connectors along an axial aperture. Thehousing upper end plug is externally prepared to receive the intendedsuspension string such as an electrically conductive wireline bail or acontinuous tubing connecting sub.

[0015] The lower end of the outer housing tube is closed with a tubularassembly that includes a stab fit nose plug. The nose plug assemblyincludes a relatively short length of heavy wall tube extending axiallyout from an internal bore plug. The bore plug penetrates the barrel ofthe housing tube end whereas the tubular portion of the nose plugextends from the lower end of the housing tube. The bore plug isperimeter sealed by high pressure O-rings and secured by a plurality ofset screws around the outside diameter of the outer housing tube.

[0016] The tubular portion of the nose plug provides a closed chamberspace for enclosing electrical conductors. The bore plug includes atubular aperture along the nose plug axis that is a load rod alignmentguide. Laterally of the load rod alignment guide is a socket for anexploding bridge wire (EBW) detonator or an exploding foil initiator(EFI).

[0017] Within the upper end of the outer housing barrel is an innertubular housing for a electronic detonation cartridge having arelatively high discharge voltage, 5,000 v or more, for example. Belowthe inner tubular housing is a cylindrical, upper detonator housing. Theupper detonator housing is resiliently separated from the lower end ofthe inner tubular housing by a suitable spring. The upper detonatorhousing includes a receptacle socket 31 for an exploding bridge wire(EBW) detonator. The axis for the upper detonator receptacle socket islaterally offset from the outer housing barrel axis.

[0018] Preferably, the severing tool structure is transported to aworking location in a primed condition with upper and lower EBWdetonators connected for firing but having no high explosive pelletsplaced between the EBW detonators. At the appropriate moment, the noseplug assembly is removed from the bottom end of the outer housing and aload rod therein removed. The upper distal end of the load rod includesa circumferential collar such as a snap ring. The opposite end of theload rod is visually marked to designate maximum and minimum quantitiesof explosive aligned along the load rod.

[0019] Explosive pellets for the invention are formed as solid cylindersections having an axial aperture. The individual pellets are stackedalong the load rod with the load rod penetrating the axial aperture. Theupper distal end collar serves as a stop limit for the pellets which areserially aligned along the rod until the lower face of the lowermostpellet coincides with the max/min indicia marking. A restriction collarsuch as a resilient O-ring is placed around the loading rod and tightlyagainst the bottom face of the lowermost explosive pellet.

[0020] The rod and pellet assembly are inserted into the outer housingbarrel until the uppermost pellet face contiguously engages the upperdetonator housing. The rod guide aperture in the nose plug is thenassembled over the lower distal end of the load rod and the lowerdetonator brought into contiguous engagement with the lowermost pelletface. The assembly is then further compressed against the loading springbetween the inner tubular housing and the upper detonator housing untilabutment between the nose plug shoulder and the lower distal end of theouter housing tube.

[0021] In the event that the invention severing tool must be disarmed,all pellets may be removed from the housing barrel as a singular unitabout the load rod. This is accomplished by removing the lower nose plugwhich exposes the lower end of the load rod. By grasping and pulling theload rod from the housing barrel, all pellets that are pinned along theload rod below the upper distal end collar are drawn out of the housingtube with the rod.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Relative to the drawings wherein like reference charactersdesignate like or similar elements or steps through the several figuresof the drawings:

[0023]FIG. 1 is a sectional view of the invention as assembled withoutan explosive charge for transport;

[0024]FIG. 2 is a sectional view of the invention with the bottom nosepiece detached from the main assembly housing;

[0025]FIG. 3 is a sectional view of an assembled, explosive pellet unit;

[0026]FIG. 4 is a sectional view of the invention with the explosivepellet unit combined with the main assembly housing but the bottom nosepiece detached therefrom;

[0027]FIG. 5 is a sectional view of the invention in operative assemblywith an explosive pellet unit.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0028] Referring to the FIG. 1 cross-sectional view of the invention 10,a tubular outer housing 12 having an internal bore 14 is sealed at anupper end by a plug 16. The plug 16 includes an axial bore 18 and anelectrical connector 20 for routing detonation signal leads 22. A boss17, projecting from the base of the plug, is externally threaded for theattachment of the desired suspension string such as an electricalwireline or service tubing.

[0029] An inner housing tube 24 is secured to and extends from the upperend plug 16 into the internal bore 14 of the outer housing 12. The innerhousing tube 24 encloses a capacitive firing cartridge 26. Below theinner housing 24 is an upper detonator housing 28. A coil spring 30links the upper detonator housing 28 to the inner housing tube 24. Anexploding bridge wire (EBW) detonator or exploding foil initiator (EFI)32 is seated within a receptacle socket formed in the upper detonatorhousing 28 laterally of the housing axis. Electrical conduits 34 connectthe capacitive firing cartridge 26 to to the EBW detonator or EFI 32.

[0030] An exploding bridge wire (EBW) detonator comprises a smallquantity of moderate to high order explosive that is detonated by theexplosive vaporization of a metal filament or foil (EFI) due to a highvoltage surge imposed upon the filament. A capacitive firing cartridgeis basically an electrical capacitator discharge circuit that functionsto to abruptly discharge with a high threshold voltage. Significantly,the EBW detonator or EFI is relatively insensitive to static or RFfrequency voltages. Consequently, the capacitive firing circuit and EBWor EFI function cooperatively to provide a substantial safety advantage.An unusually high voltage surge is required to detonate the EBWdetonator (or EFI) and the capacitive firing cartridge delivers the highvoltage surge in a precisely controlled manner. The system is relativelyimpervious to static discharges, stray electrical fields and radiofrequency emissions. Since the EBW and EFI detonation systems are,functionally, the same, hereafter and in the attached invention claims,reference to an EBW detonator is intended to include and encompass anEFI.

[0031] The lower end of the outer housing tube 12 is operatively openedand closed by a nose plug 40. The nose plug 40 comprises a plug base 42having an O-ring fitting within the lower end of the outer housing bore14. The plug base 42 may be secured to the outer housing tube 12 byshear pins or screws 44 to accomodate a straight push assembly.Projecting from the interior end of the plug base is a guide tube boss46 having an axial throughbore 48 and a receptacle socket 50 for adetonator cap 66.

[0032] Projecting from the exterior end of the plug base 42 is a heavywall nose tube 52 having a nose cap 54. The nose cap 54 may bedisassembled from the nose tube 52 for manual access into the interiorbore 56 of the nose tube 52. Detonation signal conductor leads 58 arerouted from the firing cartridge 26, through the upper detonator housingand along the wall of housing bore 14. A conductor channel 60 routes theleads 58 through the nose plug base 42 into the nose tube interior 56.This nose tube interior provides environmental protection for electricalconnections 62 with conductor leads 64 from the lower EBW detonator 66.

[0033] Although the electrical connections of both EBW detonators 32 and66 are field accessible, it is a design intent for the invention toobviate the need for field connections. Without explosive pelletmaterial in the outer housing bore 14, EBW detonators 32 and 66 are theonly explosive material in the assembly. Moreover, the separationdistance between the EBW detonators 32 and 66 essentially eliminates thepossibility of a sympathetic detonation of the two detonators.Consequently, without explosive material in the tubing bore 14, theassembly as illustrated by FIG. 1 is safe for transport with the EBWdetonators 32 and 66 connected in place.

[0034] The significance of having a severing tool that requires nodetonator connections at the well site for arming cannot be minimized.Severing tools are loaded with high explosive at the well site of use.Often, this is not an environment that contributes to the focused,intellectual concentration that the hazardous task requires.Exacerbating the physical discomfort is the emotional distractionarising from the apprehension of intimately manipulating a deadlyquantity of highly explosive material. Hence, the well site armingprocedure should be as simple and error-proof as possible. Completeelimination of all electrical connection steps is most desirable.

[0035] The load rod 70, best illustrated by FIGS. 2, 3 and 4, ispreferably a stiff, slender shaft having an end retainer 72 such as a“C” clip or snap ring. Preferably, the shaft is fabricated from anon-sparking material such as wood, glass composite or non-ferrousmetal. Individual high explosive “pellets” 74 are cylindrically formedwith a substantially uniform outer perimeter OD and a substantiallyuniform ID center bore. The term “pellets” as used herein is intended toencompass all appropriate forms of explosive material regardless of thedescriptive label applied such as “cookies”, “wafers”, or “charges”. Theaxial length of the pellets may vary within known limits, depending onthe exact weight quantity allocated to a specific pellet. The pelletsare assembled as a serial column over the rod 70 which penetrates thepellet center bore. A prior calculation has determined the maximum andminimum cumulative column length depending on the the known weightvariations. This maximum and minimum column length is translated ontothe rod 70 as an indicia band 76. The maximum and minimum lengthdimensions are measured from the rod end retainer 72. The OD of the endretainer 72 is selected to be substantially greater than the ID of thepellet center bore. Hence the pellets cannot pass over the end retainerand can slide along the rod 70 length no further than the end retainer.When loading the tool with explosive in the field, the correct quantityof explosive 74 will terminate with a lower end plane that coincideswithin the indicia band 76. An elastomer O-ring 78 constricted about theshaft of rod 70 compactly confines the pellet assembly along the rodlength.

[0036] A lower distal end portion 79 of the rod extends beyond theindicia band 76 to penetrate the guide bore 48 of the bore plug base 42when the bottom nose plug 40 is replaced after an explosive charge hasbeen positioned. This rod extension allows the high explosive to bemanually manipulated as a singular, integrated unit. In full visualfield, the explosive charge is assembled by a columned alignment of thepellets over the penetrating length of the rod. When the outside surfaceplane of the last pellet in the column aligns within the indicia band76, the lower end retainer 78 is positioned over the rod and against thelast pellet surface plane to hold the column in tight, serial assembly.Using the rod extension 79 as a handle, the explosive assembly isaxially inserted into the housing bore 14 until contiguous contact ismade with the lower face of the upper detonator housing 28.

[0037] One of the synergistic advantages to the unitary rod loadingsystem of the invention is use of lighter, axially shorter pellets, i.e.22.7 gms. These lighter weight pellets enjoy a more favoraable shippingclassification (UN 1.4S) than that imposed on heavier, 38 gm pellets (UN1.4D). In a prior art severing tool, the lighter weight pellets would beavoided due to “cocking” in the tool barrel 14 during loading. Theloading rod system of the present invention substantially eliminates the“cocking” problem, regardless of how thin the pelleet is.

[0038] With the explosive assembly in place, the lower end of thehousing is closed by placement of the nose plug 40 into the open end ofthe housing. The rod end projection 79 penetrates the guide bore 48 asthe plug base 42 is pushed to an internal seal with the housing bore 14.To assure intimate contact of the opposite end EBW detonators 32 and 66with the respective adjacent ends of the explosive assembly, the upperdetonator housing 28 is displaced against the spring 30 to accommodatethe specified length of the explosive column. Accordingly, when the noseplug 40 is seated against the end of the outer housing tube 12, both EBWdetonators are in oppositely mutual compression as is illustrated byFIG. 5. The severing tool is now prepared for lowering into a well forthe pipe cutting objective

[0039] Presently applied Explosive Safety Recommendations require thesevering tool 10 to be electrically connected to the suspension stringi.e. wireline, etc., before arming ballistically. Ballistic arming withrespect to the present invention means the insertion of the explosivePellets 24 into the housing bore 14.

[0040] On those occasions when the severing tool must be disarmedwithout discharge, it is only necessary to remove the nose plug 40 andby grasping the rod extension 79, draw the pellets 74 from the tube bore14 as a single, integrated item.

[0041] Numerous modifications and variations may be made of thestructures and methods described and illustrated herein withoutdeparting from the scope and spirit of the the invention disclosed.Accordingly, it should be understood that the embodiments described andillustrated herein are only representative of the invention and are notto be considered as limitations upon the invention as hereafter claimed.

1. An apparatus for explosively severing a length of pipe having aninternal flowbore, said apparatus comprising: a tubular exterior housinghaving an interior barrel extending between opposite distal ends of thebarrel; a plurality of high explosive pellets in axial alignment andbound together as a singular and independent unit that may beselectively inserted within said barrel and withdrawn unexplodedtherefrom as a single unit; and, electrically initiated detonation meansengaging the explosive pellet unit.
 2. An apparatus as described byclaim 1 wherein said detonation means comprises detonators at oppositeends of said pellet unit.
 3. An apparatus as described by claim 1wherein the detonators respective to said opposite ends are connectedfor simultaneous detonation.
 4. An apparatus as described by claim 3wherein said opposite end detonators resiliently bear compressivelyagainst respective ends of said pellet unit.
 5. An apparatus asdescribed by claim 3 wherein one end of said exterior housing isselectively detached, with one of said detonators, from the remainder ofsaid exterior housing for loading said pellet unit into said barrel. 6.An apparatus for explosively severing a length of pipe having aninternal flowbore, said apparatus comprising: (a) a tubular housinghaving an internal barrel space between opposite distal ends foraligning an axial column of explosive material; (b) detonator sockethousings disposed at opposite ends of said opposite distal ends; (c)resilient bias means for resiliently translating at least one sockethousing along said barrel space toward the other socket housing; (d)exploding bridge wire detonators in said socket housings; and, (e) acapacitive firing device electrically connected to said exploding bridgewire detonators.
 7. An apparatus as described by claim 6 wherein atleast one of said detonator housings may be selectively separated fromsaid tubular housings while maintaining an electrically conductiveconnection with said firing device.
 8. An apparatus as described byclaim 6 wherein one distal end of said tubular housing is sealed by aclosure means that is selectively removed from said tubular housing toload a column of explosive material into said internal barrel, saidclosure means including the socket housing respective to said one distalend.
 9. An apparatus as described by claim 8 wherein said closure meansfurther including a guide aperture for aligning said explosive materialwithin said internal barrel.
 10. An apparatus as described by claim 6wherein the socket housing respective to said other distal end isresiliently biased along the length of said internal barrel tocompressively confine said column of explosive material between saidsocket housings.
 11. A method of severing a length of pipe having aninternal flow bore comprising the steps of: assembling a plurality ofhigh explosive pellets into a singular, columned unit; depositing saidcolumned unit into a tubular barrel; resiliently engaging at least oneend of said columned unit with detonator means; positioning said tubularbarrel within said flow bore at a predetermined location along thelength of said flow bore; and, electrically initiating said detonatormeans.
 12. A method of severing a length of pipe as described by claim11 wherein detonator means engage opposite ends of said columned unit ofhigh explosive pellets.
 13. A method of severing a length of pipe asdescribed by claim 12 wherein opposite end detonator means aresimultaneously initiated.
 14. A method of severing a length of pipe asdescribed by claim 11 wherein said plurality of high explosive pelletsare unitized in a column separate from said tubular barrel and insertedin said tubular barrel as a singular unit prior to positioning saidbarrel within said flow bore.
 15. A method of severing a length of pipeas described by claim 14 wherein said plurality of pellets are formedfor meshed engagement with unitizing structure whereby said unitizingstructure and meshed pellets are inserted within or removed from saidtubular barrel as a singular unit.
 16. A method of severing a length ofpipe having an internal flow bore comprising the steps of: providing atubular barrel space for assembling a column of highly explosivematerial; providing exploding wire detonators at opposite ends of saidtubular barrel space; providing a capacitive firing device forselectively igniting said detonators substantially simultaneously;assembling a column of highly explosive material within said tubularbarrel space; resiliently engaging opposite ends of said explosivematerial column with said exploding bridge wire detonators; positioningsaid tubular barrel within the internal flow bore of a pipe at apredetermined location along the length of said flow bore; and,electrically initiating said detonator means.
 17. A method as describedby claim 16 wherein said column of explosive material is assembledexternally of said tubular barrel and positioned into said barrel spaceas an integral unit;
 18. A method of severing a string of pipe extendingwithin a well bore from a wellhead site, said method comprising thesteps of: providing a severing tool at a wellhead site, said severingtool having an internal barrel space between opposite distal ends withina substantially tubular housing; providing exploding bridge wiredetonators at said opposite distal ends; electrically connecting saidexploding bridge wire detonators to a capacitive firing device forsubstantially simultaneous ingition of said detonators by said firingdevice; delivering said electrically connected severing tool to awellhead site; depositing a column of explosive material in saidinternal barrel space between said exploding bridge wire detonators atsaid wellhead site; positioning said severing tool at a predeterminedlocation within a string of pipe suspended from said wellhead site; and,detonating said column of explosive material by an electrical signal tosaid capacitive firing device.
 19. A method as described by claim 18wherein said column of explosive material is assembled as a singularunit externally of said barrel space and deposited in said barrel spaceas a singular unit.
 20. A method as described by claim 19 wherein saidcolumn of explosive material is deposited within said barrel spacewithout electrically disconnecting either of said detonators.